1. Field of the Invention
The present invention concerns an active optical waveguide semiconductor device. A device of this kind includes a waveguide for light to be processed by the device. This waveguide is active in the sense that it is adapted to modify in a controlled manner at least one parameter characteristic of the light, using an electrical signal. The device is a semiconductor device to enable the control signal to operate by modifying a charge carrier density in the waveguide.
2. Description of the Prior Art
A device of this kind is typically an amplitude modulator, a laser emitter or amplifier, or a phase modulator, for example. It must be coupled to at least one external optical component and typically to two such components each comprising an optical fiber.
A device of this kind includes a semiconductor wafer. The wafer defines mutually perpendicular longitudinal, transverse and vertical directions. Lengths, widths and thicknesses are defined in these respective directions. The wafer has horizontal top and bottom surfaces. It extends in the longitudinal direction between two end surfaces. At least one of these end surfaces constitutes a coupling surface which an external optical component such as an optical fiber faces to achieve coupling between this external component and the device for light to be processed by the device. The wafer includes horizontal layers with a continuous crystal structure comprising layers of given thickness in succession in the vertical direction. These layers constitute structures each including one layer or a plurality of adjacent layers. The compositions, thicknesses and order of succession of the layers in a structure of this kind constitute a sequence of layers of this structure. These layers and structures are as follows, starting from the bottom surface:
A base structure having a first type of conductivity. At least an upper part of this structure constitutes a bottom confinement layer. PA1 A core structure including a high index layer having a higher refractive index increased relative to that of the surrounding materials. This high index layer has a thickness greater than that of a quantum well and a composition such that its energy gap causes it to interact with the light to be processed by the device, this interaction being conditioned by a charge carrier density in this layer, which is therefore an active layer with respect to this light. PA1 A top confinement layer which is transparent to the light to be processed by the device. This layer is formed on the core structure and with the bottom confinement layer and the core structure constitutes a guide structure. One layer of the latter is delimited in the transverse direction over at least part of its thickness to form a longitudinal strip. The light to be processed is guided along the longitudinal direction by the guide structure in a single guided mode having a given thickness. The value of said charge carrier density in the width of the strip controls interaction of the high index layer with the light of the guided mode so that this value constitutes a control density. PA1 Finally, a control layer is formed on the top confinement layer. This control layer has a second type of conductivity opposite to the first and an increased dopant concentration giving it an increased electrical conductivity to enable the control density to be varied by an electrical control signal applied between the base structure and this control layer. This increased dopant concentration causes the control layer to absorb the light to be processed.
The top confinement layer is of limited thickness so that the control signal can vary the control density. Its thickness is sufficiently great relative to that of the guided mode to limit absorption of the light in the control layer.
When included in an optical transmission or switching system, a device of this kind, such as a modulator, introduces insertion losses which are often high and which researchers have been seeking to reduce for many years. These insertion losses include Fresnel losses, internal losses and coupling losses. The Fresnel losses are caused by reflection at the entry and exit coupling surfaces of the device. They are easily and conventionally eliminated by depositing an antireflection layer on the surfaces. The internal losses result from absorption of light within the device. They are moderate because the sequence of layers of the guide structure renders a confinement ratio of this structure sufficiently high, for example greater than 70%. This confinement ratio is the ratio of the power of the guided mode within the thickness of the core structure to the total power of this mode. These losses would become high if the light had to propagate in the control layer, however. The coupling losses typically result from a mismatch between the internal light mode guided in the device and a wider external light mode guided in an external component in the form of an optical fiber. These coupling losses are conventionally reduced by using fibers fitted with lenses. Even so, the coupling losses at the end of conventional thin active structures remain high (typically 5 dB at each surface). This is why arrangements of greater or lesser complexity have been adopted to reduce these coupling losses, among other things.
An arrangement of this kind is used in a first prior art device. A monomode guide structure of this device includes a single high index layer. The structure is active given that the high index layer is active. This layer can comprise a succession of thin layers each constituting a quantum well. This active guide structure guides a mode whose thickness is chosen to achieve highly efficient interaction with limited internal losses due to absorption in the control layer. The resulting choice of thickness is too small to enable good coupling to an external optical fiber. It is typically between 0.4 .mu.m and 0.6 .mu.m. A mode of this thickness can be classified as "thin".
This can be illustrated by two typical structures that are feasible for a prior art device of this type. The high index layers of these structures are GainAsP quaternary layers between InP layers, the confinement ratio exceeding 70%. A relatively high index quaternary layer structure corresponds to a mode thickness around 0.4 .mu.m and a relatively low index quaternary layer structure to a mode thickness of 0.6 .mu.m.
These two structures and other structures in accordance with the present invention using the same materials are described hereinafter by way of their sequences of layers. Each layer will be defined between two parentheses, firstly by its refractive index n and then if necessary by its thickness e, an indication of the composition of the quaternary materials being also given by the cut-off wavelength .lambda.g of the material, i.e. the wavelength corresponding to the forbidden band of the material.
The two structures mentioned above are as follows: (n=3.17) (n=3.45 e=400 nm, .lambda.g=1 420 nm) (n=3.17) and (n=3.17) (n=3.29 e=600 nm, .lambda.g=1 110 nm) (n=3.17).
To limit coupling losses in a device of the same kind as this first prior art device, its active guide structure is followed, lengthwise of the device, by a monomode passive guide structure guiding a mode with a greater thickness. This passive structure comprises a single core layer. The thickness of the mode that it guides is chosen to limit losses where it is coupled to an external optical fiber or to some other external optical component analogous to a fiber in so far as its coupling characteristics are concerned. This thickness is typically between 1 .mu.m and 4 .mu.m. The mode having this thickness can be called "thick".
To prevent coupling losses between the two guide structures guiding the thin and thick modes, the transition between then is made highly progressive and is referred to as "adiabatic transition" or "taper". This first prior art device is described in "Efficient fiberchip butt coupling using InGaAsp/InP waveguide tapers" L. Moeil, L. Ahlers, P. Albrecht, H. Engel, H. J. Hensel, H. P. Nolting and F. Reier, (0FC/I000C'93 Technical digest, ThK2, p. 212-213 published by Optical Society of America).
A second prior art device is described in "New structure for efficiently coupling a waveguide to an optical fiber".--J. Haes, J. Willems, R. Baets, J. Buus and W. J. Stewart, (0FC/I000C'93 Technical Digest, WH8, p. 118-119 published by Optical Society of America).
An active guide structure of this second prior art device includes an active high index layer and guides a single "thin" mode. This is why, in order to limit coupling losses, it is replaced near each coupling face, without any transition area, by a composite passive structure made up of three stacked high index layers separated by dilutant layers of lower refractive index. Three modes can be guided by the respective three high index layers, with different phase velocities. All three are excited by the single thin mode, with respective initial phases. Given appropriate choices of the pertinent optical parameter values, i.e. the refractive indices and thicknesses of the layers, the three guided modes arrive at the coupling face with relative phases such that their combination is locally equivalent to a "thick" mode, enabling coupling to an optical fiber with low losses.
The arrangements adopted in the first and second prior art devices are effective in that the thin mode limits internal losses in the device and the thick mode limits coupling losses. They have the disadvantage of being costly to fabricate, however.